1. Field of the Invention
The present invention relates to an electric motor capable of reducing cogging torque.
2. Description of the Related Art
In a permanent magnet electric motor that includes a rotor having a permanent magnet, due to the presence of slots of a stator core facing an outer peripheral surface of the rotor, magnetic coenergy fluctuates during rotation of the rotor, and therefore generates cogging torque that is torque pulsation. The cogging torque is preferably reduced because the cogging torque interferes with smooth rotation of the rotor to generate sound or vibration. Conventionally, as an electric motor designed to reduce such cogging torque, there are known electric motors as described in Japanese Laid-open Patent Publication No. 2003-023740 (JP2003-023740A) and Japanese Laid-open Patent Publication No. 11-164501 (JP11-164501A).
A rotor of an electric motor described in JP2003-023740A includes a magnetic pole unit having a circular-arc outer peripheral surface bulged to an outside in a radial direction so that a waveform of a magnetic flux density generated from the rotor is a sine wave shape. Maximum outer diameter parts of the outer peripheral surface are arranged on both sides of a circumferential direction center (magnetic pole center) of the magnetic pole unit, and a concave part is formed in the circumferential direction center of the magnetic pole unit. This arrangement doubles the number of waveform peaks of cogging torque generated for each rotation of the rotor and reduces a magnitude of the cogging torque by half. On the other hand, JP11-164501A describes an electric motor in which an outer peripheral surface of a magnetic pole unit of a rotor is formed into a cylindrical shape around a rotary shaft of the rotor. This electric motor is configured such that a waveform of a magnetic flux density of the rotor is not a sine wave shape but a trapezoidal wave shape.
The electric motor described in JP2003-023740A is configured to reduce the magnitude of cogging torque by half, by substantially doubling the number of magnetic poles, but is unable to adjust the magnitude of the cogging torque to an arbitrary magnitude.